A microelectromechanical systems (MEMS) switch, and in particular a MEMS switch that operates using low actuation voltage.
A microelectromechanical system (MEMS) is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are typically formed using known integrated circuit fabrication techniques. The mechanical elements are typically fabricated using lithographic and other related processes to perform micromachining, wherein portions of a substrate (e.g., silicon wafer) are selectively etched away or added to with new materials and structural layers. MEMS devices include actuators, sensors, switches, accelerometers, and modulators.
MEMS switches (i.e., contacts, relays, shunts, etc.) have intrinsic advantages over their conventional solid-state counterparts (e.g., field-effect transistor (FET) switches), including superior power efficiency, low insertion loss and excellent isolation. However, MEMS switches are generally much slower than solid-state switches. This limitation precludes applying MEMS switches in certain technologies where sub-microsecond switching is required, such as switching an antenna between transmit and receive in high-speed wireless communication devices.
There are antenna applications where MEMS switches are critically important because of the relatively low insertion loss. One such application is in a smart antenna application that relates to switching between a plurality of antennas within a wireless communication device. Smart antenna switching applications typically require switching speeds ranging from milliseconds to seconds depending on the systems.
One type of prior art MEMS switch includes a connecting member called a xe2x80x9cbeamxe2x80x9d that is electro-thermally deflected or buckled. The buckled beam engages one or more electrical contacts to establish an electrical connection between the contacts.
FIGS. 1 and 1A illustrate a prior art MEMS switch 10 that includes a beam 12 which is electro-thermally buckled. Beam 12 is formed of a high thermal expansion conductor 14 and a low thermal expansion dielectric 16. Conductor 14 and dielectric 16 are restrained at opposing ends by anchors 18A, 18B.
Activation of MEMS switch 10 is illustrated in FIG. 1A. A voltage is applied across beam 12 such that current travels through beam 12 with much more of the current passing through low resistance conductor 14. As current passes through beam 12 (indicated by arrows A in FIG. 1A), there is resistive heating generated within beam 12 that causes beam 12 to thermally expand. The large differential between the thermal expansion of conductor 14 and dielectric 16 causes beam 12 to buckle outward toward the side of conductor 14. As beam 12 buckles, a contact stud 20 mounted on beam 12 engages contacts 22A, 22B so that signals (indicated by arrows B in FIG. 1A) can be passed between contacts 22A, 22B.
One benefit of using an electro-thermally deflected beam is that the switch requires a relatively low actuation voltage during operation. However, when the MEMS switch is in the actuated position, power is being consumed continuously in order to maintain the resistive heating within the beam.
FIG. 2 illustrates another prior art MEMS switch 30 that includes a beam 32 which is secured at opposite ends to anchors 34A, 34B. Beam 32 is secured to anchors 34A, 34B in a manner that places beam 32 under compressive stress. The compressive stress causes beam 32 to buckle. Beam 32 needs to remain in a buckled state for MEMS switch 30 to operate appropriately.
A lateral actuation electrode 36 is positioned adjacent to beam 32 at the level beam 32 would occupy were it not buckled from the compressive stress. This level of beam 32 is referred to as the neutral position and is indicated in FIG. 2 with line 38. A voltage is applied to lateral actuation electrode 36 to generate an electrostatic force that pulls beam 32 up or down toward its neutral position. The inertia of beam 32 carries it past the neutral position to the other side where beam 32 electrically connects contacts (not shown) to allow signals to pass between the contacts.
MEMS switch 30 does not require any power to maintain beam 32 in either the up or down position. One drawback associated with MEMS switch 30 is that large actuation voltages are required with electrostatic actuation in general, and in particular when electrostatic actuation is used to maneuver a buckled beam.